Semiconductor manufacturing processes are used for creating and modifying structures in substrates of semiconductor material in order to create useful devices in the substrate. Many of these manufacturing processes are conducted on substrates or wafers of the semiconductor material held in a vacuum chamber and exposed to the desired process conditions. It is well understood that the processes can be affected in a harmful way by the presence on the surface of the substrate or wafer being processed of contaminant particulates which may be derived from within the process chamber. Unwanted particulates within the process chamber can be derived from a number of sources, including mechanically moving parts, electrical interactions such as arcing with interior surfaces in the vacuum chamber, and the disturbance, e.g. during venting, of pre-existing contaminant particulates within the chamber.
Embodiments and examples of this invention will be described herein with particular emphasis on ion implantation. However, the most general principles of the invention are also relevant to other semiconductor device manufacturing processes for which the presence of contaminant particulates can be a problem.
Sensors have been proposed for detecting and counting or measuring the flux of contaminant particulates within a process environment, typically in a vacuated chamber. Generally such sensors in the prior art use optical techniques in which a light source is directed into a sample region and light which may be scattered from contaminant particulates within the sample region is detected.
U.S. Pat. No. 4,896,048, for example, describes a particle detector which may be used, for example, in the field of ion implantation. The detector may be located within the vacuum chamber and light from a source is delivered to a sample region along an optical fibre. A further bundle of fibres is used to detect scattered light and communicate the scattered light signals to a sensor outside the chamber.
U.S. Pat. No. 5,751,422 discloses an optical detector in which an optical cavity is formed having a laser medium. A sensing region is defined within the optical cavity and light scattered from contaminant particulates within the sensed region in the optical cavity is communicated to a detector.
U.S. Pat. Nos. 5,463,460 and 5,565,985 disclose a further particle monitoring sensor with provision for cleaning and preventing contamination of the windows proximate the sensing region through which light is directed from the source and through which scattered light may be detected.
None of the above prior art patent specifications describes in any detail the specific use which may be made of the signals derived from the disclosed particle detectors.
U.S. Pat. No. 5,047,648 discloses a particle detector located “in situ” in the process chamber of an ion implanter so as to detect particles flying off substrates on the scan wheel of the implanter as the substrates pass through the ion beam. A maximum flux of contaminant particulates for detection is assured by locating the detector on a tangent with respect to the scan wheel at a point where the scan wheel intersects the ion beam.
The disclosed detector is said to allow the particle level within the chamber to be monitored while wafers are still being processed, so that corrective action can be taken. No details of the nature of the corrective action are given.
In fact, hitherto in the implant art, in situ particle monitors, such as described in the above U.S. patent, have been used to provide an indication of excess particle count during a process, in order to produce an alarm to operators of the implanter so that the implant process can be “gracefully” terminated, possibly before irreparable damage is done to the wafers being processed. Corrective action can then be taken, which would typically be to perform a cleaning process on the implanter, involving repeated cycles of venting of the vacuum chamber and roughing. This cleaning process tends to dislodge particulates within the vacuum chamber and allow them to be pumped out so as to reduce the total number of particulates within the chamber.
Importantly, the corrective action known in the art involves stopping the process within the vacuum chamber, i.e. terminating the desired process conditions. Further, the corrective action would involve removing wafers from the process chamber before the above described cleaning procedure. Although in some cases it might then be possible to restart the process with the previously part processed wafers, in order to complete the processing of those wafers, this may not be practicable in many instances. The importance of prior art particle detection was to ensure that corrective action was taken before further batches of wafers are installed in the machine for subsequent processing.
Reference should also be made to the following published articles:    i) Integration of a Particle Monitor into the Control System for an Ion Implanter, Myers et al, Nuclear Instruments and Methods in Physics Research B74 (1993) pages 243–247;    ii) In Situ Particle Monitoring in a Varian E1000HP Ion Implanter, Sedgewick et al, IIT-94, pages 579 to 582;    iii) In Situ Particle Monitoring in a Varian Medium Current Implanter, Sedgewick et al, IIT-94, pages 583 to 587;    iv) Advanced In Situ Particle Monitor for Applied Materials Implanter Applications, Simmons et al, IIT-98, pages 570–573;    (v) Successful Integration of In Situ Particle Monitoring into a Volume 300 mm High Current Implant Manufacturing System, Simmons et al, IIT-2002, pages 323–326.
All the above papers disclose in situ particle monitors for use in implanters. Myers et al describes the use of such a monitor to anticipate the need to clean the process chamber and to halt a current process. When a lower threshold is exceeded an alarm is posted on completion of the current implant to allow the operator to perform maintenance. A higher threshold value being exceeded may cause the current implant to be stopped. Exceeding the higher threshold value may be the result of a so called catastrophic event, such as a wafer breakage within the chamber.
The second Simmons et al paper from IIT-2002 describes an in situ particle monitor which allows each batch of wafers being processed in a batch-type implanter to be monitored and the early detection of excess particle current. As a result, particle problems can be detected earlier and fewer batches are affected, before some corrective action is taken.
All the published papers listed above confirm the established state of the art view, that in situ particle monitors are useful for preventing a process continuing in the presence of an excessive particle count, so as to minimise the number of wafers or substrates which are improperly processed and effectively damaged.
In a further paper,
Experimental Evidence for Beam Particulate Transport in Ion Implanters, Sferlazzo et al, IIT-92, pages 565–569, an experiment is described in which artificial particulates are inserted into an ion beam and observed to be transported along the ion beam. A video camera is used to observe and record the presence of injected particles being transported by the ion beam. However, there is no discussion in this article of counting particles or measuring the flux of particles.